It has heretofore, been known to obtain carbon fibers by subjecting acrylic fibers to preoxidation in an oxidizing atmosphere (containing oxygen) at 200.degree. to 300.degree. C., preferably under tension, and then subjecting the resulting preoxidized fibers to carbonization in an inert gaseous atmosphere at 500.degree. C. or higher, preferably under tension (these methods are described in, for example, U.S. Pat. No. 4,069,297).
In these manufacturing processes, the preoxidation is an oxidation reaction. At high temperature the preoxidation can be carried out in a short period of time and is economical. However, when the preoxidation is carried out at high temperature heat is locally built up in the fibers and causes coalescence of the preoxidized fibers to one another. Carbon fibers having high qualities and high strength cannot be obtained from such preoxidized fibers.
In order to preventing coalescence, it has been proposed, for example, to adhere a cationic surface-active agent to the starting acrylic fibers (such is described in Japanese Patent Publication (unexamined) 112410/1982). To conduct preoxidation in a short period of time, it is required to effect the treatment at a higher temperature. With the above proposed technique, it is difficult to prevent coalescence of the preoxidized fibers during preoxidation at high temperature.